The present invention relates to propylene polymers incorporating macromers and a method for the preparation of branched polypropylene utilizing chiral, stereorigid transition metal catalyst compounds.
Polypropylene and related polymers are known to have low melt strength. This is a significant deficiency in key application areas such as thermoforming and blow molding. Polyethylene on the other hand is used extensively in blown film applications requiring good melt strength. The limitations in the melt strength of polypropylenes show up as excess sag in sheet extrusion, rapid thinning of walls in parts thermoformed in the melt phase, low draw-down ratios in extrusion coating, poor bubble formation in extrusion foam materials, and relative weakness in large-part blow molding. Thus, it would be highly desirable to produce polypropylene and related polymers having enhanced melt strength as well as commercially valuable processability.
Increasing the melt strength of polymers such as polypropylene has been an industrial goal for well over ten years, however, success has been limited. The desirable properties that have made low density polyethylene commercially successful are attributed in large part to high melt strength and excellent processability. Both of these properties are attributed to the presence of long chain branching which is thought to occur under high pressure polymerization conditions.
There has been some success in increasing the melt strength of polypropylene. For example, EP 190 889 A2 discloses high energy irradiation of polypropylene to create what is believed to be polypropylene having substantial free-end long branches of propylene units. EP 384 431 discloses the use of peroxide decomposition of polypropylene in the substantial absence of oxygen to obtain a similar product.
Other attempts to improve the melt properties of polypropylene include U.S. Pat. No. 5,541,236, which introduces long chain branching by bridging two PP backbones to form H-type polymers, and U.S. Pat. No. 5,514,761, which uses dienes incorporated in the backbones to achieve a similar effect. However, it is difficult to avoid cross-linking and gel formation in such processes.
Thus, there is still a need for propylene polymers having improved melt strength and good processability.
The present invention meets that need by providing a polyolefin composition consisting essentially of isotactic polypropylene and, optionally, one or more comonomers, wherein the total comonomer content of the polyolefin composition is from 0 to 20 mole percent, and further, wherein the weight average branching index gxe2x80x2 for the lower molecular weight region of the polyolefin composition is less than 0.93. Additionally, a process is provided for producing a polyolefin composition comprising:
a) contacting, in solution, at a temperature from about 90xc2x0 C. to about 120xc2x0 C., propylene monomers with a catalyst composition comprising a first chiral, stereorigid transition metal catalyst compound capable of producing isotactic polypropylene;
b) copolymerizing the product of a) with propylene and, optionally, one or more copolymerizable monomers, in a polymerization reactor under suitable polypropylene polymerization conditions using a second chiral, stereorigid transition metal catalyst capable of producing isotactic polypropylene; and
c) recovering a branched olefin polymer.